ESM cause (LTE NAS)3GPP TS 24.301 §9.9.4.4 and Annex B
What it means
Missing or unknown APN: the APN the phone asked for doesn't exist for it, or none was given and no default is subscribed.
Where you see it
In LTE NAS PDN connectivity reject, bearer resource reject or EPS bearer deactivation.
What to check
Check the APN requested by the phone against the APNs in its HSS profile and the DNS resolution of the APN FQDN.
Check the GTPv2 Create Session Response and the HSS APN configuration for the same APN.
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